A semiconductor processing apparatus often excites a process gas in a reaction chamber to form plasma by applying the RF energy provided by the RF power supply into the reaction chamber with a high vacuum environment. The plasma includes a large amount of active particles, such as electrons, ions, excited atoms, molecules, and free radicals. The active particles react physically and/or chemically with a surface of a wafer exposed to the plasma environment, thereby fulfilling etching, deposition, or other processes of the wafer. As the integrated circuit further develops, the existing technology cannot meet requirements for an etching process with a dimension of 22 nm or less. As such, a pulsed RF power supply is used as a plasma excitation source to reduce the plasma induced damage caused by continuous-wave RF energy and enlarge the process adjustment method and the process window. Currently, a key factor that restricts the development of the pulsed RF power supply as the plasma excitation source is the impedance matching technique for the pulsed RF power supply. Impedance matching refers to matching the load impedance of the pulsed RF power supply with the characteristic impedance (generally, 50 ohms) of the pulsed RF power supply. The common pulsed frequency range of the pulsed RF power supply is 100˜100 kHz, and the range of the duty cycle is 10%˜90%. Accordingly, each pulse width is only a few milliseconds. However, by using the existing impedance matching device that relies on the method of mechanical adjustment, impedance matching can hardly be fulfilled within such a few milliseconds, resulting in a low matching accuracy and a high reflection power (generally, 20%) of the pulsed RF power supply. Thus, the utilization of the pulsed RF power supply is poor.
Accordingly, an impedance matching device shown in FIG. 1 is used in existing technologies. The impedance matching device primarily uses the method of electronic adjustment, and occasionally uses the method of mechanical adjustment. Referring to FIG. 1, the impedance matching device 10 includes a control unit 11, an execution unit 12, and a matching network 13. The pulsed RF power supply 14 has a frequency-sweep function and sends a pulse synchronous signal to the control unit 11, and the pulse synchronous signal is shown in FIG. 2. During a high voltage level period, the pulsed RF power supply 14 is modulated with a RF power signal, and during a low voltage level period, the pulsed RF power supply 14 is not modulated with a RF power signal. An impedance adjustable element is disposed in the matching network 13; the pulsed RF power supply 14 automatically performs frequency-sweep matching in a high voltage level period (i.e., the pulse frequency with the maximum output power is obtained by automatic adjustment based on the load impedance of the pulsed RF power supply 14); the control unit 11 acquires a current pulse frequency of the pulsed RF power supply 14 in the high voltage level period of each pulse period in real-time based on the pulse synchronous signal, calculates a current load impedance of the pulsed RF power supply 14 according to the current pulse frequency, a circuit structure of the matching network 13 and a current position of the impedance adjustable element of the matching network 13, and determines whether the current load impedance matches the characteristic impedance of the pulsed RF power supply 14. If the current load impedance matches the characteristic impedance of the pulsed RF power supply 14, the current position of the impedance adjustable element is maintained at the low voltage level of the current pulse period. That is, the matching position is maintained. If the current load impedance does not match the characteristic impedance of the pulsed RF power supply 14, the execution unit 12 is controlled to adjust the position of the impedance adjustable element when the current pulse period is at low voltage levels, thereby performing impedance matching by adjusting the load impedance of the pulsed RF power supply 14.
FIG. 3 is a structural schematic diagram of a reaction chamber using an existing impedance matching device. Referring to FIG. 3, an induction coil 21 is disposed above the top of the reaction chamber 20, and the induction coil 21 is electrically connected to a first RF power supply 23 via a first impedance matching device 22. An electrostatic chuck 24 for bearing a wafer S is disposed in a bottom region inside the reaction chamber 20, and the electrostatic chuck 24 is electrically connected to a second RF power supply 26 via a second impedance matching device 25. The first RF power supply 23 adopts a continuous-wave signal output mode, namely, the first RF power supply 23 continuously outputs a RF power signal. The second RF power supply 26 is a pulsed RF power supply, the frequency of the RF power signal generated by the second RF power supply is 13.56 MHz, the frequency of the pulse synchronous signal is 100 Hz, and the duty cycle is 90%. The second impedance matching device 25 uses the impedance matching device illustrated in FIG. 1.
Under the aforementioned conditions, FIG. 4 is a schematic diagram of the matching status of the second impedance matching device 25 at different time points in an impedance matching process. Referring to FIG. 4, specifically, during a first pulse period: in the high voltage level period, the RF power supply 26 starts automatic frequency-sweep matching and remains in a status of “under matching”, that is, the impedance matching is not realized; in the low voltage level period, the control unit 11 performs impedance matching by controlling the execution unit 12 to adjust the position of the impedance adjustable element. During a second pulse period: in the high voltage level period, the RF power supply 26 starts the automatic frequency-sweep matching, and after a period of T, the status of “under matching” is changed to a status of “matched”, namely, the impedance matching not being realized is changed into the impedance matching is realized; in the low voltage level period, the matching position remains unchanged. The matching processes of a third pulse period and a subsequent pulse period are similar to the matching process of the second pulse period, and are not repeated herein.
Correspondingly, FIG. 5 is an impedance changing trajectory of the load impedance of the second RF power supply in the impedance matching process represented by the Smith chart. Referring to FIG. 5, the most central point of the Smith chart represents one matched resistance value (50 ohms), and the position where the most central point is located is called an impedance matching point. In such Smith chart, the impedance matching process is actually a moving process of the load impedance from an edge position of the chart towards the most central position of the chart, and the moving process specifically includes traversing the impedance zone 1, the impedance zone 2, the impedance zone 3, and the impedance zone 4 sequentially.
When the pulsed RF power supply 26 is not turned on, the load impedance is the impedance induced by an interference signal and is shown as the impedance zone 1 outside of the circle in the Smith chart. After the pulsed RF power supply 26 is turned on, in the high voltage level period of the first pulse period of the impedance matching process, the current load impedance is initially located in the impedance zone 2, the impedance value is approximately 6∠−86°, and at this moment, the pulsed RF power supply 26 has not realized ignition in the reaction chamber. As the pulsed RF power supply 26 performs automatic frequency-sweep matching, the current load impedance moves gradually from the impedance zone 2 towards the impedance matching position but has not reached the impedance matching point. In the low voltage level period of the first pulse period, no impedance matching is performed, and at this moment, the load impedance is located in the impedance zone 4 outside of the Smith circle and is the coupled signal impedance of the induction coil 21. During the second pulse period of the impedance matching process, in the high voltage level period, because the impedance adjustable element is adjusted in the low voltage level period of the first pulse period, the load impedance is, at the very beginning, located in a non-ignition impedance zone of the impedance zone 2 that moves a certain distance towards the impedance matching point. Accordingly, the pulsed RF power supply 26 does not realize ignition at first. As the pulsed RF power supply 26 performs automatic frequency-sweep matching, the current load impedance moves to the impedance zone 3, the impedance value is approximately 40∠25°, and by then, impedance matching is basically realized. In the low voltage level period of the second pulse period, the load impedance is the same located in the impedance zone 4. The movement processes of the load impedance corresponding to the third pulse period and the subsequent pulse period are similar to the movement process of the load impedance corresponding to the second pulse period, and are not repeated herein.
In practical applications, the following technical issues often exist when the aforementioned existing impedance matching device is used to perform impedance matching on the pulsed RF power supply. Because the function of the pulsed RF power supply in a processing process is to excite the process gas in the reaction chamber to form plasma, and the load impedance of the pulsed RF power supply when the reaction chamber is ignited is different from the load impedance of the pulsed RF power supply when the impedance matching is realized. Thus, after the impedance matching is realized for the first time, in the high voltage level period of each subsequent pulse period, the pulsed RF power supply needs to first realize the ignition of the reaction chamber and then realize the impedance matching. That is, in the high voltage level period of each subsequent pulse period, to realize the ignition of the process gas, the load impedance needs to be adjusted from the matched load impedance value obtained in the previous pulse period to ignition load impedance value. After ignition, the automatic frequency-sweep matching process shown in FIG. 4 and FIG. 5 may be used to match the load impedance until the load impedance value that realizes matching is achieved. In other words, when using the existing impedance matching device to realize impedance matching, the high voltage level period of each subsequent pulse period needs to undergo a relatively long automatic frequency-sweep matching period T. Thus, the matching efficiency is low, resulting in poor processing stability and low utilization of the pulsed RF power supply.
Therefore, an impedance matching method and device thereof that can implement rapid impedance matching for a pulsed RF power supply are urgently needed.